Manganese oxide thin film and oxide laminate

ABSTRACT

The present invention provides a thin film or laminate which ensures switching capabilities by phase transition of Mott transition at room temperature. An embodiment of the present invention provides a manganese oxide thin film  2  formed on a plane of a substrate  1  and having a composition represented by a composition formula RMnO 3  (where R is at least one trivalent rare earth element selected from lanthanoids), wherein an atomic layer containing an element R and not containing Mn and an atomic layer containing Mn and not containing the element R are alternately stacked along a direction perpendicular to the plane of the substrate, and the manganese oxide thin film has two nonequivalent crystal axes along an in-plane direction of the plane of the substrate. An aspect of the present invention also provides an oxide laminate having the manganese oxide thin film  2  of the above aspect to which strongly-correlated oxide thin film  3, 31  or  32  are formed contiguously.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a manganese oxide thin film and anoxide laminate. More specifically, the present invention relates to amanganese oxide thin film and an oxide laminate undergoing Motttransition by controlling temperature, electric field, magnetic field orexposure to light and undergoing switching of electrical, magnetic oroptical properties thereof.

2. Background of the Related Art

It is a recent concern that the scaling law, that provides guidance forperformance improvement of semiconductor devices, is finally imposing anend. With this concern, materials, other than the all-time used silicon,have attracted attention as allowing novel operating principles. Forexample, in the field of spintronics incorporating the spin degree offreedom, development is underway for realizing high-density non-volatilememories which allow high speed operation as fast as DRAM (dynamicrandom access memories).

Meanwhile, studies on strongly-correlated electron system materials havealso been advanced to which the band theory, the fundamental theory ofsemiconductor device design, cannot be applied. During the studies,materials have been found which undergo huge and rapid changes inphysical properties resulting from phase transition of electron systems.In strongly-correlated electron system materials, not only the spindegree of freedom but also the orbital degree of freedom is involved inthe state of a phase of the electron systems, and thus variouselectronic phases emerge with different ordering formed by spin,electron and orbital. A typical example of a strongly-correlatedelectron system material is perovskite-type manganese oxides which areknown to have electron systems exhibiting a charge-ordered phase inwhich 3d electrons of manganese (Mn) are in order due to a first-orderphase transition or an orbital-ordered phase in which orbitals are inorder.

In the charge-ordered phase and orbital-ordered phase, the electricresistance is high due to carrier localization and thus the electronicphase is insulating. The electronic phase has a magnetic property whichcorresponds to an antiferromagnetic phase due to superexchange anddouble exchange interactions. There are many cases, however, in whichthe electronic states of charge-ordered phases or orbital-ordered phasesshould be regarded as semiconducting, because although carriers in thecharge-ordered phases and orbital-ordered phases are localized, theresistance is lower than that of so-called band insulators. However, theelectronic phase of charge-ordered phases and orbital-ordered phases areherein referred to as an insulator phase as is customary. To thecontrary, the electronic phase having a low resistance and a metal-likebehavior exhibits a ferromagnetic phase due to aligned spins. Althoughthere are various definitions for the metallic phase, the metallic phaseherein refers to “the phase having a positive temperature differentialcoefficient of resistivity”. Correspondingly, the insulator phase can bere-defined as “the phase having a negative temperature differentialcoefficient of resistivity”.

It is disclosed that the phenomena in which various switchingcapabilities emerge are observed in single-crystal bulk materials havingany of electronic phases among the charge-ordered phase, theorbital-ordered phase and a phase in which both charge and orbital areordered, i.e., a charge and orbital ordered phase, (see Japanese PatentApplication Laid-open Nos. H8-133894, H10-255481, and H10-261291(respectively, Patent Documents 1 to 3). These phenomena are typicallyobserved as a huge change in resistance or transition between anantiferromagnetic phase and a ferromagnetic phase. For example, a changein resistance by orders of magnitude in response to a magnetic fieldapplication is well known as the colossal magnetoresistive effect.

In order to prepare practical devices such as electronic devices,magnetic devices, as well as optical devices, which utilize thesephenomena for exhibiting switching capabilities, it is required toeffectuate the phenomena causing the switching capabilities in atemperature range at or above room temperature (e.g., 300 K or above).However, the switching capabilities disclosed in Patent Documents 1 to 3are all confirmed at a low temperature such as at or below the liquidnitrogen temperature (77 K). The perovskite-type manganese oxidedisclosed in these Patent Documents is a laminate in which, providedthat the chemical composition is designated as ABO₃, atomic layers arerepeatedly stacked such as an AO layer, a BO₂ layer, an AO layer and soon. The crystal structure of such a laminate is herein represented asAO—BO₂-AO. In the perovskite unit cell, an A site, a B site and an O(oxygen) respectively occupy the vertex, the body centre and the facecentre. Manganese is located at the B site.

In Patent Documents 1 to 3, the type of the element or ion whichoccupies the A sites of the perovskite crystal structure is consideredto be involved in a decrease in the temperature at which the switchingphenomenon is observed, i.e., at which the charge-orbital orderingappear in the perovskite-type manganese oxide (hereinafter referred toas “appearance temperature”). Simply stated, the appearance temperatureis decreased because of the random occupation of the A sites of theperovskite crystal structure by cations of trivalent rare earth(hereinafter designated as “R”) and divalent alkaline earth (“Ae”). Tothe contrary, it is also known that the transition temperature to thecharge-ordered phase can be raised to about 500 K if the elements orions at the A sites are ordered to be AeO—BO₂—RO—BO₂-AeO—BO₂—RO—BO₂— . .. . Hereinafter “A site ordering” refers to regular arrangement of ionsat A sites such as those disclosed herein and “A-site orderedperovskite-type manganese oxide” refers to the perovskite-type manganeseoxide with A site ordering. A group of materials which exhibits such ahigh transition temperature is characterized in that the materialscontain Ba (barium) as an alkaline earth (Ae). It has been reportedthat, for example, the oxides containing Ba as an alkaline earth (Ae)and Y (yttrium), Ho (holmium), Dy (dysprosium), Tb (terbium), Gd(gadolinium), Eu (europium) or Sm (samarium), which have lower ionicradii, as a rare earth element (R) have a transition temperature aboveroom temperature.

In order to effectuate devices such as electronic devices, e.g.,magnetic devices, as well as optical devices, which utilize thesephenomena, it is required to prepare the perovskite-type manganese oxidein a thin film form and effectuate the switching phenomenon. However,there has been a problem such that the switching capabilities aredifficult to be obtained when the thin film is formed on a(100)-oriented substrate, because the lattice deformation, referred toas Jahn-Teller mode which is required for phase transition to thecharge-ordered phase or the orbital-ordered phase is suppressed due tothe in-plane 4-fold symmetry.

On the other hand, Japanese Patent Application Laid-open No. 2005-213078(Patent Document 4) discloses the formation of a perovskite oxide thinfilm utilizing a (110)-oriented substrate. According to the disclosureof Patent Document 4, when the in-plane 4-fold symmetry is broken in the(110)-oriented substrate, shear deformation of the crystal lattice ispermitted upon switching of the formed thin film. When shear deformationoccurs, the crystal lattices are oriented parallel to the substrateplane while the charge-ordered plane or the orbital-ordered plane isnon-parallel to the plane of substrate surface.

Japanese Patent Application Laid-open No. 2008-156188 (Patent Document5) also discloses an example of the A-site ordered perovskite-typemanganese oxide which is in the form of a thin film. This disclosurereports an application and light irradiation method in which, once anamorphous thin film is deposited, laser annealing is carried out forcrystallization and A-site ordering. It is actually confirmed byelectron diffraction that A-sites are in order in a SmBaMn₂O₆ thin filmformed on a (100)-oriented SrTiO₃ substrate (lattice constant: 0.3905nm).

However, the A-site ordered perovskite-type manganese oxide has such aproblem that the degree of A site ions order significantly affects thetemperature at which the switching phenomenon is effectuated, i.e., theappearance temperature of charge-orbital ordering. Particularly in thecase of a thin film of the A-site ordered perovskite-type manganeseoxide, the degree of A site ion order may be reduced even with anintroduction of defects in the formed thin film or a slight departure inthe composition of the thin film. Moreover, the thin film on the(110)-oriented substrate reported in Patent Document 4 has such aproblem that it does not contribute to any of reduction in the degree oforder and reduction in the appearance temperature. Thus conventionalperovskite-type oxide thin films have unsolved problems regarding thelow appearance temperature of charge-orbital ordering and the appearancetemperature of charge-orbital ordering that is at or above roomtemperature depending on the degree of A site order and the resultinginstability.

SUMMARY OF THE INVENTION

The present invention has been accomplished in view of the above issues.The present invention provides a manganese oxide thin film or an oxidelaminate which exhibits switching capabilities by controlling phasetransition with an external stimulation (external field) at roomtemperature, in order to contribute to creation of novel devices.

As a result of investigation towards the above problems, the presentinventor assumed that the above problems are, first of all, caused bythe fact that the A sites of the perovskite-type Mn oxide are occupiedby two types of cations, i.e., cations of a trivalent rare earth element(R) and a divalent alkaline earth element (Ae, e.g., Sr or Ba). Thepresent inventor came to the realization that an approach in which aperovskite-type Mn oxide is used in which two types of cations occupythe A sites cannot solve the above problems and thus investigated adifferent approach, thereby finding a specific means for solving theabove problems.

The present invention solves at least one of the above problems based ona completely new principle. As the specific means for solving theproblems, an aspect of the present invention provides a manganese oxidethin film formed on a plane of a substrate and having a compositionrepresented by composition formula RMnO₃, where R is at least onetrivalent rare earth element selected from lanthanoids, wherein anatomic layer containing an element R and not containing Mn, and anatomic layer containing Mn and not containing the element R arealternately stacked along a stacking direction perpendicular to theplane of the substrate, and the manganese oxide thin film has twononequivalent crystal axes along the in-plane direction of the plane ofthe substrate. The term “two nonequivalent crystal axes” means twocrystal axes which are asymmetric against an in-plane, 4-fold symmetricoperation. For example, two in-plane axes in a cubic (100) substrate are[010] and [001] and these cannot be differentiated after an in-plane,4-fold symmetric operation, namely, rotation at 90 degrees. In thiscase, two crystal axes are said to be equivalent. On the other hand, twoin-plane axes in a cubic (210) substrate are [−120] and [001]. They donot match after 4-fold symmetric operation and thus two crystal axes arereferred to as nonequivalent.

The manganese oxide thin film of the present aspect is a thin film madeof perovskite-type manganese oxide. The manganese oxide has a crystallattice having a composition represented by ABO₃. The crystal of themanganese oxide thin film of the present aspect contains, asconventional perovskite-type crystals, Mn (manganese) at the B sites andhas oxygen octahedrons surrounding Mn. In the crystal of the manganeseoxide thin film of the present aspect, the A sites are occupied only bycations of the trivalent rare earth element (R). Thus, contrary to theconventional crystals, no divalent alkaline earth (Ae) is located at theA sites. The rare earth element R of the present aspect is typically alanthanoid trivalent rare earth element, i.e., at least one elementselected from the group consisting of La (lanthanum), Ce (cerium), Pr(praseodymium), Nd (neodymium), Pm (promethium), Sm, Eu, Gd, Tb, Dy, Ho,Er (erbium), Tm (thulium), Yb (ytterbium) and Lu (lutetium).

The substances disclosed in Patent Documents 4 and 5 have divalentalkaline earth (Ae) elements such as Sr (Patent Document 4) or Ba(Patent Document 5) at the A sites.

In the manganese oxide thin film of the above aspect of the presentinvention, the atomic layer containing an element R without Mn and theatomic layer containing Mn without the element R are alternately stackedalong a direction perpendicular to the plane of the substrate. In themanganese oxide of this aspect, the atomic layer containing the elementR without Mn typically is a RO layer, namely a layer containing elementsR and O (oxygen). On the other hand, the atomic layer containing Mnwithout the element R is typically a MnO₂ layer, namely a layercontaining Mn and O. When these layers are in the above typicalconfiguration, the RO layer and the MnO₂ layer alternately stacked inthe direction perpendicular to the plane of the substrate are believedto be charged to +1 and −1, respectively. As a result, to the crystal ofthe manganese oxide thin film of the above aspect is always appliedvoltage or an electric field due to the charged polar surface, becausethe nominal valences of the elements are +3 for R, −2 for O and +3 forMn due to the charge neutrality condition of RMnO₃. When such anelectric field is generated, the reduction in an external field requiredfor exhibition of insulator-metal transition can be expected. Inaddition, the manganese oxide thin film of the above aspect has twononequivalent crystal axes along the in-plane direction of the plane ofthe substrate. Thus the symmetry of the crystal of the manganese oxidein-plane of the substrate is lower than 4-fold symmetry to permit sheardeformation, thereby allowing the first-order phase transition.According to the disclosure of Patent Document 5, because of the use ofthe (100)-oriented SrTiO₃ substrate, the crystal lattice of theSmBaMn₂O₆ thin film formed on the substrate has 4-fold symmetry whichdoes not permit shear deformation. Alternatively, if a (100) orientedmanganese oxide is formed which corresponds to a thin film containing ROlayers and MnO₂ layers alternately stacked in the directionperpendicular to the plane of the substrate, two equivalent crystal axesare formed along the in-plane direction of the plane of the substrate.Therefore the crystal of the (100) oriented manganese oxide is 4-foldsymmetric even with the RO layers and MnO₂ layers. Contrary to thisstructure, the manganese oxide thin film of the above aspect has twononequivalent crystal axes along the in-plane direction of the plane ofthe substrate and thus does not include, for example, a (100) oriented4-fold symmetric manganese oxide thin film. The phrase “has twononequivalent crystal axes along the in-plane direction of the plane ofthe substrate” means that two equivalent crystal axes do not exist alongthe in-plane direction of the plane of the substrate.

The electronic phase of the manganese oxide which is a material of thethin film of the aspect of the present invention exhibits such aproperty that it undergoes phase transition between insulator and metal,namely Mott transition. Such manganese oxide is a member of a group ofsubstances which are generally referred to as Mott insulators. However,the manganese oxide thin film of the present application is a thin filmwhich exhibits a property that can undergo metal-insulator transition,and does not always have the insulator phase. Such a material ishereinafter referred to as “manganese oxide”. Not only temperature butalso an external stimulation (hereinafter referred to as “externalfield”) may be generally involved in Mott transition. In Mott transitionwhich is generated only by temperature without application of anexternal field, the insulator phase and the metallic phase respectivelyappear in low and high temperature ranges. On the other hand, in Motttransition which is generated at a certain temperature with an alteredexternal field, the insulator phase and the metallic phase respectivelyappear in the ranges with weak and strong external fields. The manganeseoxide thin film of the aspect of the present invention can undergo Motttransition at room temperature (e.g. 300 K) with an external field. Thismeans in the above aspect either that the transition temperature of Motttransition is decreased that is otherwise higher than a room temperaturein general or that a threshold of an external field required for Motttransition is reduced than the conventional threshold, or both. Theexternal field herein typically includes magnetic field, electric field,electric current, light, pressure and any combinations thereof.

According to an aspect of the present invention, the manganese oxidethin film according to the above aspect is provided wherein thecomposition of the manganese oxide thin film is represented by thecomposition formula RMnO₃ (where R is at least one trivalent rare earthelement selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm,Eu, Gd, Tb and Dy).

The above group of the trivalent rare earth elements corresponds to agroup of lanthanoid elements in order of atomic number excluding theelements Ho and thereafter. The trivalent rare earth element selectedfrom the above group is advantageous because the degree of rotation ofthe oxygen octahedron can be controlled and thus the degree ofexhibition of orbital ordering can be adjusted.

According to an aspect of the present invention, the manganese oxidethin film according to the above aspect is provided wherein thecomposition of the manganese oxide thin film is represented by thecomposition formula RMnO₃ (where R is at least two trivalent rare earthelements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm,Eu, Gd, Tb and Dy).

The cation of the trivalent rare earth element R is not necessarily ofone type in the composition formula RMnO₃ of the manganese oxide thinfilm of the present invention. Namely, the chemical composition of thecomposition which is manganese oxide of the present aspect representedby RMnO₃ where two trivalent rare earth elements are used as the elementR can alternatively represented by (R₁MnO₃)_(x)(R₂MnO₃)_(1-X), whereinR₁ and R₂ respectively represent distinct rare earth elements that canprovide trivalent cations and 0<X<1. The composition expressed as aboveis typically a solid solution which contains the manganese oxide R₁MnO₃containing the rare earth element R₁ and the manganese oxide R₂MnO₃containing the rare earth element R₂ at an arbitrary ratio of X:1−X. Inany case regardless of the expressions, the problems resulting fromvariation in the degree of order in the crystal of the manganese oxidethin film are in principle solved in the present aspect, because onlythe cations of the trivalent rare earth elements are located at the Asites.

In the manganese oxide thin film of the present aspect, the trivalentrare earth element R comprises multiple elements having different ionicradii. This affects the rotation of the oxygen octahedron in the crystalstructure, leading to two effects. One of the effects is such that theaverage lattice constant of the crystal lattice of the manganese oxideformed into a thin film can be altered. The other effect is such that avariation (randomness) is introduced to the rotation angle of the oxygenoctahedron of the crystal lattice of the manganese oxide. Both of theseact so as to reduce the threshold of an external field for Motttransition. Therefore use of multiple rare earth elements R is apreferable aspect. The same is true for the use of three or moretrivalent rare earth elements R. These mechanisms are described indetail in section “1-5. Two effects of multiple elements R” below.

According to an aspect of the present invention, the manganese oxidethin film according to the above aspect is provided wherein a materialforming the manganese oxide thin film has, as a bulk substance, thecubic root of the unit cell volume of the crystal lattice that is lowerthan the lattice constant of the crystal lattice of the substrate.

According to this aspect, the orbital ordered plane is disposeddiagonally to the plane of the substrate and tensile strain is appliedto the manganese oxide thin film. As a result, the angle of the bondMn—O—Mn in the crystal lattice of the manganese oxide is increased toapproach to 180 degrees. Because of this, increased carrier transfer inMn—O—Mn, allows easy switching from the insulator phase to the metallicphase in Mott transition. This may cause such an effect that, when Motttransition is generated by means of an external field, the intensity ofthe external field required for Mott transition can be decreased.

In addition, according to an aspect of the present invention, themanganese oxide thin film according to the above aspect is providedwherein the orientation of the substrate is (210)orientation.

According to this aspect, epitaxial growth utilizing the atomic stackedplane of the substrate can be carried out, allowing a single crystalthin film without defects such as misfit. Further, the RMnO₃ thin filmon the (210)-oriented substrate has a generated polarization in thein-plane [1-20] axis direction slightly leaning to the directionperpendicular to the plane due to the symmetry breaking. Thus in thepresent aspect, voltage (electric field) is endogenously applied, inaddition to the direction perpendicular to the plane, to the in-planedirection and therefore a depolarization field due to polarization isalso applied along the in-plane direction and the threshold of anexternal field for insulator-metal transition is reduced.

Further, an aspect of the present invention also provides an oxidelaminate which includes an additional layer. Namely in an aspect of thepresent invention is provided an oxide laminate including: the manganeseoxide thin film according to any of the above aspects; and astrongly-correlated oxide thin film contiguous to the manganese oxidethin film, wherein the total thickness t of the oxide laminate, thethickness tm of the manganese oxide thin film and the thickness t1 ofthe strongly-correlated oxide thin film satisfy the following relationrelative to the critical thickness tc under which thestrongly-correlated oxide thin film is to be a metallic phase:t=tm+t1>tc and t1<tc.

In the oxide laminate of the present aspect, the strongly-correlatedoxide thin film is disposed so as to be contiguous to and stacked withthe manganese oxide thin film. The crystal structure of a materialforming the strongly-correlated oxide thin film is of perovskiterepresented by ABO₃ similar to the manganese oxide thin film. However,different from the manganese oxide thin film, the A sites of the crystallattice of the strongly-correlated oxide thin film are not alwaysoccupied only by cations of a trivalent rare earth element (R). Theoxide laminate of the present aspect allows easier detection ofswitching capabilities of the manganese oxide thin film due toinsulator-metal transition (Mott transition) compared to the abovemanganese oxide thin film alone, because the switching capabilities ofthe manganese oxide thin film due to insulator-metal transition (Motttransition), i.e., the change in the electron status, can be easilydetected externally as, for example, the change in resistance of anoxide laminate specimen. The mechanism for this facilitation ofdetection may be referred to as dimensional crossover and is describedin detail in the section “1-7. Improvement in detectability by stacking(dimensional crossover)” below. It is further possible, as a result ofthe facilitation of detection, to secondarily decrease the threshold ofan external field required for transition. This is partly because thethickness of the manganese oxide thin film undergoing Mott transitioncan be decreased compared to the case of the manganese oxide thin filmalone.

An aspect of the present invention also provides an oxide laminateincluding: the manganese oxide thin film according to any of the aboveaspects; a first strongly-correlated oxide thin film contiguous to onesurface of the manganese oxide thin film; and a secondstrongly-correlated oxide thin film contiguous to the other surface ofthe manganese oxide thin film, wherein the total thickness t of theoxide laminate, the thickness tm of the manganese oxide thin film andthicknesses t1 and t2 respectively of the first and secondstrongly-correlated oxide thin films satisfy the following relationrelative to the critical thickness tc under which thestrongly-correlated oxide thin film is to be a metallic phase:t=tm+t1+t2>tc and max(t1, t2)<tc, where max( ) is the function whichreturns the maximum value among variables.

In the oxide laminate of the present aspect, the strongly-correlatedoxide thin films are disposed so as to be contiguous to both surfaces ofthe manganese oxide thin film. The effect obtained by contacting thestrongly-correlated oxide thin film can be further significantlyenhanced compared to the case where the strongly-correlated oxide thinfilm is provided only at one surface.

The manganese oxide thin film or the oxide laminate of any aspects ofthe present invention contains a rare earth element(s) R having aconstant valence of +3 at the A sites and the atomic layer containingthe element R without Mn and the atomic layer containing Mn without theelement R which are alternately stacked along the directionperpendicular to the plane of the substrate, and thus in principle isnot affected by variation in the degree of order and allows Motttransition controlled by an external field at room temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic section view of a manganese oxide thin film of anembodiment of the present invention in which FIG. 1(A) is an overallview showing the configurations of the manganese oxide thin film formedon a substrate and FIG. 1(B) and FIG. 1(C) show an enlarged view showingatomic stacked planes of the manganese oxide thin film (section view);

FIG. 2 is an illustration view explaining an additional electric fieldgenerated in a manganese oxide thin film in an embodiment of the presentinvention in which FIG. 2(A) and FIG. 2(B) are section views similar toFIGS. 1(A) and 1(B), respectively;

FIG. 3 is a schematic section view showing an exemplary configuration ofan oxide laminate containing a manganese oxide thin film prepared so asto contact with a strongly-correlated oxide thin film of an embodimentof the present invention in which FIG. 3(A) shows an example that astrongly-correlated oxide thin film is formed on the manganese oxidethin film on the side of a substrate, and FIG. 3(B) shows an examplethat a strongly-correlated oxide thin film is formed on the top surfaceof the manganese oxide thin film;

FIG. 4 is a schematic section view of an exemplary oxide laminatecontaining a manganese oxide thin film and strongly-correlated oxidethin films formed on both surfaces thereof in an embodiment of thepresent invention;

FIG. 5 is an illustration view showing that the orbital ordered plane ina manganese oxide thin film of an embodiment of the present invention isthe (100) plane;

FIG. 6 is an illustration view showing the angle of Mn—O—Mn in amanganese oxide thin film of an embodiment of the present invention.FIG. 6( a) shows the state where, in the crystal lattice, the oxygenoctahedron is deformed in the direction of the rotation, so that theangle of Mn—O—Mn is lower than 180 degrees; and FIG. 6( b) shows thestate where, in the crystal lattice, the oxygen octahedron is deformedin the direction of the rotation due to tensile strain from thesubstrate, so that the angle of Mn—O—Mn is increased; and

FIG. 7 is a schematic view showing, in a manganese oxide thin film of anembodiment of the present invention, the difference in strain of oxygenoctahedrons in the crystal structure where two lanthanoid elements R₁and R₂ having different ionic radii are randomly arranged.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the manganese oxide thin film according to thepresent invention are hereinafter described by referring to thedrawings. In the following descriptions, common parts or elements in allfigures are designated by common reference numerals unless otherwisestated. The respective elements in the embodiments are not necessarilydepicted to scale.

First Embodiment 1. Basic Principle 1-1. Facilitation of Mott Transitionin Manganese Oxide Thin Film

An embodiment of the manganese oxide thin film according to the presentinvention is hereinafter described based on the drawings. First, a basicprinciple for providing switching capabilities at room temperature,i.e., a basic principle for effectuating Mott transition in themanganese oxide thin film at room temperature by means of an externalfield is explained. Generally, the manganese oxide thin film has anextremely higher orbital ordering temperature than that of A-siteordered Mn oxides and the like. For example, PrMnO₃ has the orbitalordering temperature of as high as 1000 K or higher. Namely, themanganese oxide thin film at room temperature of about 300 K, forexample, is in the orbital ordered state. This is one of importantpoints to be acknowledged.

In the present example, the element at the site of Pr (A site) of PrMnO₃is now replaced by the elements in order of lanthanoid such as La, Ce,Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. The above orderof lanthanoid is so-called “Lanthanide contraction” and represents thedescending order of ionic radius. When the element at the A site isreplaced with sequentially from La, the orbital ordering temperature ofthe manganese oxide thin films increases while the antiferromagnetictransition temperature decreases. This tendency continues up to Dy. Whenthe element is replaced with the following Ho, the orbital orderingtemperature drops compared to Dy and the antiferromagnetic transitiontemperature starts to increase again. In this case, the oxygenoctahedron surrounding Mn is deformed so as to have a GdFeO₃ typedistorted structure, namely so as to be rotated in the crystal latticewithout deforming the lattice of the A sites. The degree of thistranslocation or rotating deformation increases while replacinglanthanoid from La to Ho in this order. When replacing the elementswithin the range from Er to Lu, the crystal structure of a bulksubstance is likely to be hexagonal rather than orthorhombic because theionic radius is further decreased. However, in the form of a thin film,the orthorhombic structure can be achieved by allowing epitaxial growthon a cubic perovskite-type substrate. Therefore all elements oflanthanoid can be employed as the elements to be replaced at the Asites. As a result, the angle θ of Mn—O—Mn (FIG. 6) decreases duringreplacement of the elements in this order. FIG. 6 is an illustrationview showing the angle of Mn—O—Mn in the manganese oxide thin film ofthe present embodiment. FIG. 6( a) shows the state where the crystallattice is deformed in the direction of rotation of the oxygenoctahedron, so that the angle of Mn—O—Mn is lower than 180 degrees andFIG. 6( b) shows the state where the crystal lattice is deformed in thedirection of rotation of the oxygen octahedron due to tensile strainfrom the substrate, so that the angle θ of Mn—O—Mn is increased. Theangle θ which is lower than 180 degrees affects the band width, which isan indicator of conductivity of carriers, so as to deteriorate theconductivity, because the angle θ significantly affects the degeneracybetween the 2p orbital of O²⁻ and the e_(g) orbital that is generated bycrystal field splitting of the 3d orbital of Mn³⁺.

Thus the present inventor believes that the lanthanoid elementsexcluding Ho and thereafter, i.e., La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tband Dy allow the orbital ordering governed by the same physicalmechanism. Within the range of the elements, the liability of theappearance of the orbital ordering, as the antiferromagnetic transitiontemperature, systematically depends on the ionic radius.

Next, the reason for one of the phenomena described above in the sectionof Background Art as problems, i.e., the reason why the temperature ofinsulator-metal transition by an external field remains low is nowdescribed. This phenomenon occurs because, simply stated, the order ofthe electronic phase is too “robust” to transit the electronic phase ofmanganese oxide to the metallic phase by applying an external field thatis generally available. Namely, the threshold of an external fieldrequired to decrease the order of the electronic phase in manganeseoxide to obtain the metallic phase is extremely high. When reviewingconventional approaches in light of the above, the understanding of theprinciple which has been conventionally relied on can be facilitated.Namely, in manganese oxide which could otherwise be a metal according tothe band theory but is a Mott insulator due to the electron correlation,the conventional principle is employed such that the above “robustness”is decreased by doping holes so as to intentionally weaken the electroncorrelation. Actually, all conventional modes for controlling Motttransition by external fields rely on this principle. However, as far asrelying on this principle, a trade-off cannot be avoided such that theorbital ordering temperature and the charge-orbital ordering temperatureare below room temperature. The fact is that the conventional approachescould not overcome this trade-off.

In light of this situation, the present inventor raised the question asto which mechanism governs the robustness of the order of the electronsystem in Mott insulators such as manganese oxide. While trying toaddress this question, the present inventor came up with a hypothesisthat the robustness of the orbital ordering is caused by cooperativephenomena and depends on the number of electron orbitals. If thishypothesis is correct, the above trade-off could be highly possiblyaddressed even for manganese oxide, which has a too “robust” electronsystem to be addressed in the form of a bulk crystal, only whenmanganese oxide is formed into a thin film and the number of orbitals issufficiently reduced. Thus, the present inventor reached the idea thatwhen manganese oxide is made into the form of a thin film, the above“robustness” could be reduced to such a degree that the “robustness”could be controlled by external fields. This concept provided the ideafor the present invention.

With regard to the probability of the above hypothesis, experimentaldescriptions are provided in Examples hereinbelow. The additionaltheoretical description supporting the above hypothesis is now provided.The phenomena observed in a strongly-correlated electron system such ascharge ordering and orbital ordering are cooperative phenomena andaspects of the many-body effect in substances having high electroncorrelation effect. Namely, when only one unit cell is a subject thatcontains only one Mn³⁺ ion having the 3d orbital, the definition ofcharge or electron orbital ordering is not applicable. Therefore theelectron status of the system containing two unit cells in series isconsidered. In this case, the state of the electron orbital of one unitcell (orbital state) and the orbital state of the other unit cell are ina competitive state. Thus the orbital ordered state is achieved if thesystem is more stable with the ordered electron orbitals and the orbitalordered state collapses if the system is more stable without the orderedelectron orbitals. Actually, there is a possibility that the energydifference between these two states, i.e., the state where the orbitalordered state is achieved and the state where the orbital ordered statecollapses, may be too low to render the system of two unit cells to takeeither of the states. Therefore it is not pronounced that the orbitalordering is reached for the electron state of the system containing twounit cells in series.

Meanwhile, a system is considered which contains N unit cells (N is aninteger which is sufficiently bigger than 2). In this case, compared tothe state where the orbital of only one unit cell among N unit cells isdifferent from the orbitals of N−1 unit cells, it is more stable thatall orbitals in N unit cells are ordered. Thus the surrounding N−1 unitcells produce interaction with one unit cell so as to order thedifferent orbital. Further, even compared to the state where theorbitals of a few unit cells among N unit cells are different from otherunit cells, it is more stable that all orbitals in N unit cells areordered. Accordingly, in the system containing N unit cells that issufficiently bigger than 2, the interaction is produced between theorbitals of unit cells so as to order all orbitals and thus the wholesystem is stabilized.

It is revealed from the above properties of two unit cells and N unitcells that the stability of the electron system depends on the size ofthe electron system, namely the number of unit cells. Again, both chargeordering and orbital ordering are derived from cooperative phenomena insubstances having a strong electron correlation, and thus the orderingsare said to be products of self assembly of electrons. Therefore thefact that the number of unit cells is high plays a substantial role forcharge ordering and orbital ordering. Thus the above hypothesis that therobustness in the orbital ordering which is a cooperative phenomenondepends on the number of the electron orbitals is theoretically andsufficiently reasonable.

1-2. Selection of Crystal Structure

In order to allow switching capabilities in manganese oxide thin films,in addition to reduction of the threshold of external fields forallowing switching capabilities, the fact that Mott transition is afirst order transition is taken into account. Thus the manganese oxideof the present embodiment employs the crystallite symmetry that permitsshear deformation, so that it is not an obstacle to the Jahn-Teller modetransition. More specifically, the manganese oxide thin film of thepresent embodiment has the crystal structure which comprises atomicstacked planes of alternate RO layers and MnO₂ layers stacked in thedirection perpendicular to the plane of the substrate, i.e., the crystalstructure where the layers are stacked as RO—MnO₂—RO . . . . FIG. 1 is aschematic section view of an exemplary manganese oxide thin film havingthe RMnO₃ structure of the present embodiment and shows a section viewof the manganese oxide thin film formed on a plane of the (210)-orientedsubstrate. FIG. 1( a) is an overall view showing the configurations ofthe manganese oxide thin film formed on the substrate, FIG. 1( b) is asection view along a plane perpendicular to the [001] axis and FIG. 1(c) is a section view along a plane perpendicular to the [1-20] axis.FIGS. 1( b) and 1(c) both show the crystal structures of the manganeseoxide thin film sectioned at a plane perpendicular to the plane of thesubstrate.

In the crystal structure containing atomic stacked planes where ROlayers and MnO₂ layers are stacked alternately, RO atomic layers andMnO₂ atomic layers are alternately stacked and disposed, as shown inFIGS. 1( b) and 1(c), so as to form atomic layers parallel to the planeof the substrate (not shown in FIGS. 1( b) and 1(c)) which extends alongthe horizontal direction on the plane of paper. Particularly, FIGS. 1(b) and 1(c) exemplify the cases where the crystal structure of theperovskite structure of manganese oxide thin film of the presentembodiment represented by the composition formula RMnO₃ is cubic. It isfirst of all understood from the crystal structure of the manganeseoxide thin film of the present embodiment that two crystal axes in-planeof the substrate are nonequivalent. Therefore the manganese oxide thinfilm of the present embodiment allows shear deformation and thus thefirst order transition. Actually, the symmetry in the (210) plane is1-fold symmetric and thus insulator-metal transition can be exhibited byMott transition.

However, the material of the manganese oxide thin layer of the presentembodiment, i.e., manganese oxide having the perovskite structure andrepresented by the composition formula RMnO₃ may have the perovskitestructure in the crystal structure with the crystal lattice other thancubic, i.e., only with a lower order symmetry such as tetragonal,orthorhombic, monoclinic, triclinic, trigonal and hexagonal. This isbecause in any of these cases, shear deformation is permitted when theRO atomic layers and the MnO₂ atomic layers are alternately stacked andtwo in-plane crystal axes are nonequivalent as the manganese oxide thinfilm of the present embodiment. The perovskite structure of the presentembodiment encompasses substances having a crystal structure such thatonly a plurality of unit cells connected in series can provide thefundamental unit lattice of the crystal lattice as described above. Thecrystal structures shown in FIGS. 1( b) and 1(c) can be confirmed byidentification of crystallographic point groups according to well knownX-ray diffraction. Particularly the fact that the RO atomic layers andthe MnO₂ atomic layers are alternately stacked can be confirmed bydirect observation of atoms with a STEM (scanning transmission electronmicroscope).

Next the electric properties of the atomic layers in the crystalstructures shown in FIGS. 1( b) and 1(c) are described. As describedabove, a stacked structure of RO—MnO₂—RO—MnO₂— . . . is obtained in thedirection perpendicular to the plane of the substrate and the atomiclayer containing the element R without Mn (RO atomic layer) and theatomic layer containing Mn without the element R (MnO₂ atomic layer) arealternately stacked. When the valence of the rare earth R is assumed tobe generally stable at +3 and the valence of O be as −2 assuming anionic bond, the valence of Mn is +3 due to the charge neutralitycondition of RMnO₃. When the charge distribution of the atomic layersforming the laminate of the crystal structure is reviewed under theseassumptions, it is noticed that RO is charged to +1 and MnO₂ is chargedto −1, namely the atomic layers are alternately charged to + and −.These symbols are shown in FIGS. 1( b) and 1(c). As a result, themanganese oxide thin film 2 of the present embodiment has a polarsurface.

1-3. Selection of Substrate

The unfilled arrow in FIG. 1( a) shows voltage (electric field)endogenously applied from the polar surface. When the manganese oxidethin film 2 is allowed to grow on the substrate 1 of which compositionis expressed as ABO₃ and the surface of the substrate 1 at which themanganese oxide thin film 2 is formed has a BO₂ atomic layer at the end,namely the surface of the substrate 1 is the BO₂ plane, the first atomiclayer of the manganese oxide thin film 2 starting to grow is the ROlayer and thus voltage (electric field) is oriented to the direction ofthe unfilled arrow in FIG. 1( a). Similarly, when the surface of thesubstrate 1 has an AO plane at the end, the direction of the arrow isreversed. There is no difficulty in controlling the layer at the end ofthe surface of the substrate 1.

As described above, utilizing a polar surface in order to alternatelystack the atomic plane containing R and the atomic plane containing Mnso as to be RO—MnO₂—RO—MnO₂— . . . along the direction perpendicular tothe plane of the substrate is one of typical procedures that aregenerally applicable to manganese oxide thin films. One of the mosttypical polar surfaces is, as described above, a (210)-orientedsubstrate employed as the substrate 1. The crystal structures shown inFIGS. 1( b) and 1(c) can be formed by forming the crystal structure ofthe manganese oxide thin film having the composition formula RMnO₃ so asto be coherent relative to the crystal of the substrate 1.

The present inventor also found that the effect of employing thesubstrate of (210)-orientation having a low symmetry as the substrate 1is that not only an electric field perpendicular to the plane of thesubstrate but also another electric field is generated. FIG. 2 is anillustration view explaining an additional electric field in the presentembodiment. FIGS. 2( a) and 2(b) are section views similar to FIGS. 1(a) and 1(b), respectively. The arrows attached to the elements R and Mnin the atomic stacked plane shown in FIG. 2( b) show the directions ofrelative translocation of R and Mn in the actual crystal lattices.Namely, according to the investigation by the present inventor, in theactual crystal lattices obtained by forming the manganese oxide thinfilm 2 on the substrate 1 corresponding to the substrate of(210)-orientation having a low symmetry, the positions of R and Mnappear to be translocated as shown in FIG. 2( b) in the crystal latticesof manganese oxide forming the manganese oxide thin film 2. Thistranslocation corresponds to the one in which positively charged cationsR and Mn are translocated relative to O (valence of −2), therebyaccompanying polarization. The polarization is induced in the directionof the [−120]axis slightly leaning to the stacking direction, namely inthe direction of the [−110] axis. In FIG. 2( a), the arrow in themanganese oxide thin film 2 shows the macroscopic direction ofpolarization generated in the manganese oxide thin film 2 as a wholeresulting from the polarization generated inside of the manganese oxide.As described above, by employing the (210)-oriented substrate as thesubstrate 1, the macroscopic polarization can be generated. This effectcannot be obtained in a thin film, for example, on a (100)-orientedsubstrate having in-plane 4-fold symmetry. In addition, in the manganeseoxide thin film of the present configuration, endogenous voltage(electric field) is acting in both thickness direction and in-planedirection and thus it is expected that the threshold of an externalfield required for insulator-metal transition is further reduced. As aresult of the translocation described above, R and Mn are slightlytranslocated relative to O in the RO atomic layers and the MnO₂ atomiclayers. Therefore the planes formed with cations are misaligned from theplanes formed with oxygen in the RO atomic layers and the MnO₂ atomiclayers. The above translocation is generated regardless of theelectronic phase being a metallic phase or an insulating phase; howeverthe polarization due to the translocation is screened in a metallicphase while the effect of the polarization is apparent in an insulatingphase.

1-4. Utilization of Substrate Strain

In the present embodiment, transition to the metallic phase can befacilitated by utilizing strain applied to the manganese oxide thin film2 from the substrate 1. This mechanism involves the disposition of theoxygen octahedron surrounding Mn in manganese oxide, a Mott insulator,represented by the composition RMnO₃. As described above, the oxygenoctahedron is accompanied by the GdFeO₃ type distorted structure(gradient translocation or rotation deformation). However, thedeformation of the oxygen octahedron can be reduced by applying tensilestrain resulting from substrate strain to manganese oxide of themanganese oxide thin film 2. In order to reduce the deformation, aspecific combination of the compositions of manganese oxide and thesubstrate 1 may be selected so as to obtain the cubic root of the unitcell volume in a bulk substance of manganese oxide undergoing Motttransition that is lower than the lattice constant of the substrate 1.Thereby the orbital ordered plane can be obtained which is the (010)plane, namely is inclined at about 45 degrees relative to the plane ofthe substrate. FIG. 5 is an illustration view showing that in themanganese oxide thin film of the present embodiment, the orbital orderedplanes shown with chained lines are the (010) planes. In thisconfiguration, substrate strain acts so as to bring the angle of Mn—O—Mntowards linear (180 degrees), namely increase the band width of thecarriers (electrons). The difference is shown in FIG. 6( b) incomparison with the state before receiving the action of the substratestrain (FIG. 6( a)). As a result, the external field required forswitching can be reduced for the increased amount of the band width.

1-5. Two Effects of Multiple Elements R

As described above, the element R may be not only one but also multipletrivalent rare earth elements selected from the group consisting of La,Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb and Dy. When two trivalent rare earthelements are used as R, for example, the composition of manganese oxiderepresented by RMnO₃ of the above aspect also encompasses thecomposition represented by (R₁MnO₃)_(X)(R₂MnO₃)_(1-X), wherein R₁ and R₂respectively represent distinct rare earth elements that can providetrivalent cations and 0<X<1. The composition expressed as above is, asdescribed above, a solid solution which contains the manganese oxideR₁MnO₃ containing the rare earth element R₁ and the manganese oxideR₂MnO₃ containing the rare earth element R₂ at an arbitrary ratio ofX:1−X. By employing multiple trivalent rare earth elements R, there aremultiple ionic radii of cations located at the A sites in manganeseoxide. This affects the rotation of the oxygen octahedron, resulting intwo effects.

1-5-1. Adjustment of Lattice Constant by Means of Differential IonicRadii.

The first effect is adjustment of the lattice constant of manganeseoxide. In other words, it may be the modulation of the lattice constantof manganese oxide by the ratio among multiple rare earth elements R.The lattice constant of manganese oxide having the composition formula(R₁MnO₃)_(X)(R₂MnO₃)_(1-X) is, in average, a weighted average obtainedfrom the lattice constants of the respective crystal lattices of themanganese oxide R₁MnO₃ containing the rare earth element R₁ and themanganese oxide R₂MnO₃ containing the rare earth element R₂ inconsideration of the composition ratio X:1−X. In this case, R₁ and R₂are selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb and Dy so as tohave different ionic radii each other. Accordingly, the lattice constantsuch as the cubic root of the unit cell volume described in the section“1-4. Utilization of substrate strain” can be adjusted as an average.Thus the lattice constant of manganese oxide can be determined byaveraging various ionic radii of multiple elements R, resulting inadjustment of strain applied from the substrate 1 to manganese oxide inthe form of a thin film. As a result, during gradient translocation orrotation deformation of the GdFeO₃ type, deformation of the oxygenoctahedron can be reduced. Namely, the action of the substrate strainwhich affects the angle of Mn—O—Mn can be modulated by employingmultiple rare earth elements R, resulting in reduction of the thresholdof an external field required for Mott transition.

1-5-2. Randomness Introduced to Rotational Angle of Oxygen Octahedron.

The second effect results from the phenomenon such that the rotationalangles of the oxygen octahedrons in crystal lattices are varied whenmultiple rare earth elements R are located depending on the positionsthereof. It can be said that multiple rare earth elements R are locatedin order to intentionally introduce the variation in the crystallattices. FIG. 7 is a schematic view showing, in the manganese oxidethin film 2, the difference in strain of oxygen octahedrons in thecrystal structure where two lanthanoid elements R₁ and R₂ havingdifferent ionic radii are randomly located. In this figure, forsimplification, the positions of cations are fixed at the cubicconfiguration and change in the positions of oxygen atoms is shown. Thedeformation of the Jahn-Teller mode is the deformation of the oxygenoctahedron surrounding Mn. Therefore, as shown with chained lines inFIG. 6, oxygen atoms are affected by whether the nearby lanthanoid is R₁or R₂. As a result, the carriers (electrons) are affected by thefluctuated angle of Mn—O—Mn during passing through a number of crystallattices. The fluctuation leads to reduction in the threshold of anexternal field required for Mott transition. Two experimental facts andtheoretical explanations for helping understanding of the relationshipbetween the fluctuation and Mott transition are added hereinbelow.

1-5-2-1. First Experimental Fact.

The first experimental fact is provided for a conventional perovskitemanganese oxide in which atoms having different valences are randomlylocated at A sites. The perovskite manganese oxide having thecomposition Pr_(0.5)Ca_(0.5)MnO₃ is manganese oxide in which Pr and Carespectively having the nominal valences of +3 and +2 randomly occupythe A sites. The manganese oxide has checkerboard-like sequences of Mnwith nominal valence of 3 and Mn with nominal valence of 4 in alternatemanner within a crystal plane at or below 240 K, and shows acharge-orbital ordered insulating phase where electron orbitals arealigned. However, when the temperature is increased, manganese oxide isconverted to the paramagnetic insulating phase. In this substance, theexperimental fact is known that when Mn having nominal valence of +4 isreplaced by chemically stable Cr with nominal valence of 3, thecharge-orbital ordered insulating phase is facilitated to be degradedand the metallic phase is facilitated to be exhibited. This is the firstexperimental fact. Trivalent Cr randomly replacing the positions of Mncan be regarded as quadrivalent Mn of which site is fixed. Thephenomenon relating to manganese oxide exhibiting the charge-orbitalordered insulating phase can be explained to be occurred because therandomness introduced at the B sites after replacement of Mn⁴⁺ with Cr³⁺prevents a long distance order of the charge-orbital ordered phase and aferromagnetic metallic phase in the charge-orbital ordered phase isgenerated to facilitate transition of the electron system to themetallic phase.

1-5-2-2. Second Experimental Fact.

The second experimental fact is a phenomenon which is more directlyobserved when the randomness is decreased in the conventional A siteorder. The perovskite manganese oxide represented by the compositionformula Sm_(0.5)Ba_(0.5)MnO₃ is known to have two crystal structures.One structure has the A site order and the other does not. In thecrystal system with (100) orientation has the series of atomic layers inthe former lattice structure represented byBaO₂—MnO₂—SmO₂—MnO₂—BaO₂—MnO₂— . . . while in the latter structure, itis (Ba, Sm)O₂—MnO₂—(Ba, Sm)O₂— . . . wherein (Ba, Sm)O₂ is an atomiclayer where Ba and Sm randomly occupy the A sites. It is known that theformer structure with the A site order has a higher temperature T_(OO)at which the orbital ordered phase disappears than that of the latterstructure without the A site order. This is the second experimentalfact. This experimental fact means that the crystal structurecorresponding to the A site order which decreases entropy directlyincreases the degree of order of the electron system. To the contrary,it can be said that the randomness introduced at the A sites has adirect effect to decrease the degree of order of the electron system.

1-5-2-3. Effect of Randomness to Electron System.

According to the above first and second experimental facts andtheoretical explanations that support these facts, the second effect,i.e., the effect of variation (randomness) introduced into therotational angles of the oxygen octahedrons wherein multiple rare earthelements R are located can be at least qualitatively predicted. Namely,by intentionally introducing variation to the crystal lattices, thethreshold of an external field required for Mott transition may bereduced. Particularly, by taking into account the strong relationshipbetween the angle of Mn—O—Mn and the conduction band width of thecarriers, the degree of the reduction in the threshold may besufficiently observed, resulting in enhances utility thereof.

From the view point of the object for reducing the threshold of anexternal field for Mott transition, it is not always necessary tostrictly differentiate the first effect from the second effect. When itis required to differentiate these effects by experimental confirmation,the following experiment may be carried out. Two kinds of manganeseoxide are prepared; for example one contains one rare earth element(e.g. Sm) and the other contains two rare earth elements (e.g. Pr andTb) with both having the same lattice constant (average latticeconstant). The transition temperature and the threshold of the externalfield for Mott transition are then compared. Upon this comparison, theratio between two rare earth elements Pr and Tb is adjusted to obtainthe same lattice constant as the manganese oxide containing one rareearth element Sm, and then reduction in the transition temperature orthreshold can be judged to be the result of contribution of the secondeffect.

1-5-3. Feasibility of Multiple Rare Earth Elements.

Importantly, all elements in the element group of La, Ce, Pr, Nd, Pm,Sm, Eu, Gd, Tb and Dy have the stable valence of +3 with an exception ofCe, which also has the stable valence of +3 as one of the stablevalences and therefore able to have the electron configuration as atrivalent cation. This means, among others, that the value X in thecomposition (R₁MnO₃)_(X)(R₂MnO₃)_(1-X) is not strictly limited as far asR₁ and R₂ are selected from the above range of rare earth elements. Thisapplies to the composition of the thin film of manganese oxide as wellas the composition of the target material for forming the thin film. Theabove-mentioned also means that the properties of the electron system ofmanganese oxide are not directly affected by the difference in charge orelectron orbital which is different from the conventional cases wheredivalent cations affect the A site order in terms of electron orbitals.Based on the above properties, the rare earth elements R₁ and R₂ and theratio X can be selected with mainly focusing to adjust the effect on theband width resulting from the difference in ionic radii and theresulting strain in the crystal structure and to adjust geometricproperties of the crystal lattice such as matching of the latticeconstant with the substrate 1 and introduction of the randomness. Thiscircumstance similarly applies to not only where only two rare earthelements R are used but also where all combinations of two or moreelements R selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb and Dy areused.

1-6. External Detectability and Effectuation of Device Functionality

It is possible to detect whether any of the manganese oxide thin filmsprovided by the present embodiment actually undergoes Mott transition byvarious analysis means. For example, the presence of transition can bemeasured by optical measurements or measurements of transmittance orreflectance as the change in the electron structure corresponding to theenergy of probe light for measurements. Mott transition can also bedetected as arbitrary physical quantities such as magnetic properties,deformation or electric resistance. The change in physical quantitiescan provide not only detectability of Mott transition but also materialproperties which are utilized as switching capabilities upon applicationof the manganese oxide thin film of the present embodiment to devices.

1-7. Improvement in Detectability by Stacking (Dimensional Crossover)

However, in some cases, external detection of the change in materialproperties may be difficult when Mott insulators such as manganese oxideare prepared in the form of a thin film as above. This problem may notalways occur. When this problem occurs in relation to the Drudecomponent, namely the direct current resistant component, which isrepresented by electron conductivity reflecting electric properties, itcan be said that the carriers (electrons) are localized resulting fromthe low dimensionality of the system (two dimensionality in case of thethin film). In case of a thin film, for example, electrons may belocalized at a portion of the two dimensional region to decreaseconductivity. In order to address this problem, it is preferable toutilize a device called dimensional crossover in the present embodiment.

A thin film formed contiguously to the manganese oxide thin film, namelya strongly-correlated oxide thin film which is formed so as to besuccessive to the manganese oxide thin film is now described. Thestrongly-correlated oxide thin film is a layer which is different frommanganese oxide undergoing Mott transition and is added to exploitdimensional crossover. In general, it is preferable that astrongly-correlated oxide thin film has a thickness higher than acertain level in order to stabilize the metallic phase or effectuatemetal-insulator transition, because the strongly-correlated oxide thinfilm having an extremely low thickness may have difficulty ineffectuating the stable metallic phase or metal-insulator transition.Thus when a strongly-correlated oxide thin film has a higher thicknessthan a certain critical thickness (hereinafter referred to as “criticalthickness”), the strongly-correlated oxide can have the metallic phaseor undergo metal-insulator transition. In this sense, the criticalthickness may be regarded as the lower limit of the thickness of thestrongly-correlated oxide in order to obtain the stable metallic phaseor exhibit metal-insulator transition. The configuration of an oxidelaminate in which the strongly-correlated oxide thin film and themanganese oxide thin film are formed contiguously or successively eachother on a substrate is now reviewed. FIG. 3 is a schematic section viewshowing an exemplary configuration of the oxide laminate containing themanganese oxide thin film prepared contiguously to thestrongly-correlated oxide thin film of the present embodiment. FIG. 3(a) shows an example of the strongly-correlated oxide thin film formed onthe manganese oxide thin film on the side of the substrate and FIG. 3(b) shows an example of the strongly-correlated oxide thin film formed onthe top surface of the manganese oxide thin film. The oxide laminate mayencompass the configurations in which the strongly-correlated metal thinfilm is first formed on the plane of the substrate and then themanganese oxide thin film is formed thereon (FIG. 3( a)) and in which,conversely, the manganese oxide thin film is first formed on the planeof the substrate and then the strongly-correlated metal thin film isformed thereon (FIG. 3( b)). In this embodiment, the total thickness tof the oxide laminate, the thickness tm of the manganese oxide and thethickness t1 of the strongly-correlated oxide thin film satisfy therelation relative to the critical thickness tc of the metallic phase forthe strongly-correlated oxide thin film of: t=tm+t1>tc and t1<tc. Forexample, the critical thickness tc of a strongly-correlated oxide thinfilm (assumed to have the metallic phase at room temperature) of aferromagnetic metal, a La_(0.7)Sr_(0.3)MnO₃ thin film, on the(210)-oriented substrate is 8 unit cells (about 4 nm).

When the layers are formed so as to satisfy the above relationship ofthe thickness, upon Mott transition corresponding to insulator-metaltransition of manganese oxide by application of an external field, thecarriers localized in the strongly-correlated oxide thin film feel thethickness of t=tm+t1 rather than t1 which had been felt, wherein t1 isless than tc and t exceeds tc. Thus when the manganese oxide thin filmundergoes Mott transition and is changed from insulator to metal, theeffect thereof is also reflected to the transition of thestrongly-correlated oxide thin film, resulting in an improvement indetectability. This is the principle of dimensional crossover. Byexploiting dimensional crossover, the change in electric resistance dueto Mott transition can be extracted as electric current. Therefore evenwhen the manganese oxide thin film is prepared, in order to realize theswitching with a weak external field, so as to have a thickness lessthan the lower limit required for securing the detectability by electriccurrent, the detection with electric current is possible with the aid ofthe strongly-correlated oxide thin film. The carriers in the Mottinsulator of course have an effect for improving the detectability byenhancing the electric current.

More preferably, in order to further effectively utilize dimensionalcrossover, the strongly-correlated oxide thin film is provided on bothsurfaces, instead of one surface, contiguously to the manganese oxidethin film to form the oxide laminate. FIG. 4 is a schematic section viewof an exemplary oxide laminate containing a manganese oxide thin filmand strongly-correlated oxide thin films contiguous to both surfacesthereof in the present embodiment. Namely, the total thickness t of theoxide laminate, the thickness tm of the manganese oxide thin film, thethickness t1 of the first strongly-correlated oxide thin film 31 and thethickness t2 of the second strongly-correlated oxide thin film 32satisfy the relation relative to the critical thickness tc of themetallic phase for the strongly-correlated oxide thin film of:t=tm+t1+t2>tc and max(t1, t2)<tc, wherein max( ) is the function whichreturns the maximum value among variables. When the strongly-correlatedoxide thin films are formed contiguously to both surfaces of themanganese oxide thin film, the effect by dimensional crossover isfurther effectively exhibited. Thus the thickness tm of the manganeseoxide thin film can be further reduced compared to the case where thestrongly-correlated oxide thin film is disposed only on one surface.Accordingly switching by a further weak external field can be realized.

2. Examples

The present embodiments are now described based on specific Examples.The materials, amount of use, proportion, processes, order ofprocedures, orientation of elements or members, specific dispositionsand external fields employed for measurements and the like can beappropriately modified unless they depart from the scope of the presentinvention. Thus the scope of the invention is not limited to thefollowing specific examples. The descriptions are made by furtherreferring to FIGS. 3 and 4.

2-1. Example 1

Example 1 for the present embodiment is an example of an oxide laminateprepared to have the configurations shown in FIG. 4 comprising first andsecond strongly-correlated oxide thin films 31 and 32 contiguous to bothsurfaces of a manganese oxide thin film 2. The manganese oxide thin film2 used was TbMnO₃, the first and second strongly-correlated oxide thinfilms 31 and 32 used were La_(0.5)Sr_(0.5)MnO₃ (hereinafter designatedas LSMO) and the substrate 1 used was (210)-oriented (LaAlO₃)_(0.3)(SrAl_(0.5)Ta_(0.5)O₃)_(0.7) (hereinafter designated as LSAT) substrate.The surface of this LSAT (210) substrate has a B site at the end. Thematerial of the manganese oxide thin film 2, TbMnO₃, has the cubic rootof the unit cell volume as a bulk substance of 0.3853 nm which is lowerthan the lattice constant, 0.387 nm, of LSAT of the substrate 1, andthus the orbital ordered plane in Example 1 is the (010) plane and it isexpected that tensile strain may be applied to the manganese oxide thinfilm 2 from the substrate 1. The first and second strongly-correlatedoxide thin films 31 and 32 were, instead of La_(0.7)Sr_(0.3)MnO₃ whichhas a similar composition as LSMO and has maximum Curie temperatureT_(c) (370 K), LSMO which has an increased amount of overdoped Sr. It isintended to increase the Curie temperature T_(c) of the first and secondstrongly-correlated oxide thin films 31 and 32 by taking into accountthe carriers (electrons) supplied upon insulator-metal transition of themanganese oxide thin film 2.

The method for preparing the oxide laminate of Example 1 is firstdescribed. The manganese oxide thin film 2, the first and secondstrongly-correlated oxide thin films 31 and 32 were formed by laserablation. Target materials for the respective thin films used werecylindrical moulded articles having φ20 mm×5 mm of polycrystallinematerials of the respective materials prepared by solid reaction. TheLSAT (210) substrate was attached in a vacuum chamber which was thenvacuum exhausted to 3×10⁻⁹ Torr (4×10⁻⁷ Pa) or lower. Highly pure oxygengas (1 mTorr (0.133 Pa)) was introduced to the chamber and the substratewas heated so as to reach the achieving temperature of 900° C. In orderto obtain the surface having the B site at the end of the surface of theLSAT (210) substrate, etching of the surface with buffered hydrofluoricacid is preliminarily carried out. The LSMO target was irradiated with aKrF excimer laser having the wavelength of 248 nm introduced through alaser introduction port of the chamber to obtain the firststrongly-correlated oxide thin film 31 corresponding to only 15 atomiclayers of LSMO. The atomic layer herein corresponds to the one in whichone atomic layer has the (210) plane distance d (210). The filmthickness or the number of atomic layers was controlled according to thepreliminary determination based on the relationship between the numberof shots of laser pulse and the number of atomic layers. Subsequently inthe same atmosphere, the TbMnO₃ target was irradiated with the laserthrough the port to obtain the manganese oxide thin film 2 correspondingto only 6 atomic layers of TbMnO₃. The LSMO target was again used toobtain the second strongly-correlated oxide thin film 32 correspondingto only 15 atomic layers of LSMO. The first strongly-correlated oxidethin film 31 has the thickness t1 of 5 unit cells (about 2.6 nm), themanganese oxide thin film 2 has the thickness tm of 2 unit cells (about1.1 nm) and the second strongly-correlated oxide thin film 32 has thethickness t2 of 5 unit cells (about 2.6 nm). The oxide laminate has thetotal thickness t of 6.3 nm. The critical thickness tc of LSMO in thefirst and second strongly-correlated oxide thin films 31 and 32 requiredfor LSMO being a metallic phase at room temperature (300 K) is 8 unitcells (4.1 nm). Thus the thicknesses of the layers in the oxide laminateprepared in the present Example satisfy the relationship oft=tm+t1+t2>tc and max(t1, t2)<tc.

The prepared oxide laminate containing the manganese oxide thin film 2was provided with a 4-terminal electrode and magnetoresistancemeasurements were carried out at room temperature (300 K). The magneticfield was employed as the external field because the measurement iseasy. In the measurements, the resistance value of the specimen startedto decrease by application of a magnetic field having the magnetic fluxdensity of 4.2 T or more and was decreased to 10 kΩ under a magneticfield of 4.8 T. Thus the huge negative magnetoresistant effect wasconfirmed. The subsequent reduction of the magnetic field resulted inrestoration of the resistance of 10 MΩ or more, demonstrating that Motttransition corresponding to insulator-metal transition was exhibited inthe manganese oxide 2 in the oxide laminate at room temperature. Asdescribed above, it was demonstrated that the manganese oxide thin film2 which allows switching at room temperature can be obtained.

2-2. Example 2

In Example 1, an example of the oxide laminate was described comprisingthe strongly-correlated oxide thin films contiguous to both surfaces ofthe manganese oxide thin film. However, dimensional crossover can beutilized even when an oxide laminate is used in which thestrongly-correlated oxide thin film is contiguous to only one surface ofthe manganese oxide thin film. In order to confirm this point, Example 2of the present embodiment is described which is an oxide laminate havinga two-layer structure similar to the one shown in FIG. 3( a). In Example2, the substrate 1 used was an LSAT (210) substrate, thestrongly-correlated oxide thin film 3 corresponding to only 21 atomiclayers of LSMO was prepared and the manganese oxide thin film 2 wasformed thereon corresponding to only 9 atomic layers of TbMnO₃. Thepreparation process of deciding on an atomic layer at the end of thesurface of the substrate 1 and the oxide laminate of Example 2, namelythe manganese oxide thin film 2 and the strongly-correlated oxide thinfilm 3, were the same as those in Example 1.

The specimen prepared as Example 2 was provided with a 4-terminalelectrode and the resistance in the plane was measured withoutapplication of a magnetic field. During the course of increasingtemperature from low (below liquid nitrogen temperature),insulator-metal transition was first observed at about 200 K. Thisresults from insulator-metal transition of LSMO which is thestrongly-correlated oxide thin film 3. Subsequent increase of thetemperature resulted in transition of the whole specimen into aninsulator at the temperature range (253 to 353 K) including roomtemperature (300 K) at which electronic devices are supposed to operate.Magnetoresistance was measured by the similar manner as Example 1 andthe specimen of Example 2 behaved to show 1 kΩ under a magnetic fieldwith the magnetic flux density of 5 T and 100 kΩ without application ofa magnetic field. Thus the magnetoresistance at room temperature ofExample 2 was lower than the specimen of Example 1 under application ofa magnetic field while the increase in the resistance withoutapplication of a magnetic field was rather small with the change inresistance being as low as within 2 digits. This change in resistancewas sufficiently detectable; however it is ideally further greater. Thepresent inventor believes that the change in resistance was low becauseof leakage current from LSMO which is the strongly-correlated oxide thinfilm 3 formed to have a high thickness.

Similar magnetoresistance effect was measured with the configurationshown in FIG. 3( b) with the reversed order of the thin films on thesubstrate, namely a specimen prepared by preparing the manganese oxidethin film 2 corresponding to TbMnO₃ on the side of substrate 1 and thenpreparing the strongly-correlated oxide thin film 3 corresponding toLSMO.

2-3. Example 3

Example 3 was an experiment to confirm the reason as to why a highmagnetoresistant effect was not obtained with the two-layer structure ofExample 2. In Example 3, a specimen was prepared having a lower totalthickness than that of the oxide laminate of Example 2. One of objectsof Example 3 is to confirm the speculation that the reason of smallchange in resistance in Example 2 was the increased leakage currentresulting from a high thickness of the strongly-correlated oxide thinfilm 3 corresponding to LSMO. Specifically, the specimen of Example 3was prepared by preparing, on the substrate 1 corresponding to an LSAT(210) substrate, the strongly-correlated oxide thin film 3 correspondingto only 15 atomic layers of LSMO that were fewer than Example 2 and thenpreparing the manganese oxide thin film 2 corresponding to only 12atomic layers of TbMnO₃ that was thicker than Example 2. Accordingly,contrary to Example 2, the specimen of Example 3 did not showmagnetoresistance effect at all in the range from without magnetic fieldto a magnetic field of 5 T. This is believed to be due to an increase inthe thickness of the manganese oxide thin film corresponding to theTbMnO₃ layer. In the 4-terminal measurement, the distance (electrodedistance) between two electrodes for application of current at both endsamong four electrodes linearly aligned with certain distances waschanged from 500 μm as in Example 2 to 5 Resistance was measured bycontrolling the amount of current applied to the manganese oxide thinfilm 2 and using an intermediate pair of electrodes for voltagemeasurement. The resistance was 100 MΩ with application of 0.1 μA butwas decreased to as low as 1 kΩ with application of 40 μA, demonstratingthat the change in resistance was 5 digits or more between appliedelectric current of 0.1 μA and 40 μA. The measurement was also carriedout under simultaneous application of electric current and magneticfield, i.e., 6 μA was applied while a magnetic field with the magneticflux density of 5 T was applied. This time, it was revealed that themagnetoresistance effect was measured. Therefore it is revealed thatinsulator-metal transition can be obtained at room temperature byapplication of multiple external fields.

Modified Examples of the Present Embodiment

The present embodiment can be carried out with the configurations of themanganese oxide thin film and the oxide laminate other than thoseexplicitly described including Examples 1 to 3. For example, thesubstrate employed may be SrTiO₃ (210) as well as various tetragonalperovskite substrates other than the LSAT (210) substrate (Examples 1 to3) or the LSAT (210) substrate (Examples 2 and 3). In order to adjustthe relationship of the lattice constant between the formed manganeseoxide and the substrate, a solid solution may be used which containsmultiple elements having the same valence (+3) at the A sites ofmanganese oxide. Examples thereof may include solid solution having thecomposition Pr_(1-x)Nd_(x)MnO₃ (0<x<1) or the like, namely containingPrMnO₃ and NdMnO₃ at the ratio 1−x:x. Particularly PrMnO₃ and NdMnO₃form solid solution in whole composition range at any ratio. Thereforethe composition ratio of the manganese oxide thin film can be adjustedby preparing a target at a desired ratio by laser ablation similar tothe above Examples. As described above, Examples 1 to 3 exploitdimensional crossover by means of the configuration of the oxidelaminate in order to facilitate external detection of Mott transition.However, switching capabilities per se by controlling Mott transition atroom temperature with an external field are realized even in the case ofthe manganese oxide thin film without forming a laminate.

The embodiments of the present invention have been specificallydescribed hereinabove. The embodiments and Examples are described forthe purpose of illustration of the present invention, and materials andcompositions of the thin film and the substrate, film thickness,formation method thereof, type and application method of the externalfield and the like exemplified in the present embodiments are notlimited to the above embodiments. The scope of the invention of thepresent application is rather defined by the descriptions in the claims.Modified examples within the scope of the present invention includingother combinations of the above embodiments are also encompassed by theclaims.

The present invention provides a manganese oxide thin film or an oxidelaminate for allowing switching capabilities by a Mott transition atroom temperature controlled by an external field and has application fordevices utilizing the switching phenomenon by means of application of anexternal field such as magnetic field, light, electricity or pressure.

1. A manganese oxide thin film formed on a plane of a substrate,comprising: a substrate of plane orientation (210); and a manganeseoxide thin film formed on a plane of the substrate and having acomposition represented by composition formula RMnO₃, where R is atleast one trivalent rare earth element selected from lanthanoids,wherein an atomic layer containing an element R and not containing Mn,and an atomic layer containing Mn and not containing the element R arealternately stacked along a stacking direction perpendicular to theplane of the substrate, the manganese oxide thin film has twononequivalent crystal axes along an in-plane direction of the plane ofthe substrate and has a polar surface containing an electric field, andcations R and Mn are translocated relative to O to generate polarizationso as to lean toward the stacking direction.
 2. The manganese oxide thinfilm according to claim 1, wherein R is at least one trivalent rareearth element selected from the group consisting of La, Ce, Pr, Nd, Pm,Sm, Eu, Gd, Tb and Dy. 3-7. (canceled)
 8. A manganese oxide thin filmformed on a plane of a substrate, comprising: a substrate; and amanganese oxide thin film formed on a plane of the substrate and havinga composition represented by composition formula RMnO₃, where R is atleast two trivalent rare earth elements selected from the groupconsisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb and Dy, wherein anatomic layer containing an element R and not containing Mn, and anatomic layer containing Mn and not containing the element R arealternately stacked along a stacking direction perpendicular to theplane of the substrate and the manganese oxide thin film has twononequivalent crystal axes along an in-plane direction of the plane ofthe substrate.
 9. A manganese oxide thin film formed on a plane of asubstrate, comprising: a substrate having a crystal lattice and alattice constant; and a manganese oxide thin film formed on thesubstrate and having a composition represented by composition formulaRMnO₃, where R is at least one trivalent rare earth element selectedfrom lanthanoids, wherein an atomic layer containing an element R andnot containing Mn, and an atomic layer containing Mn and not containingthe element R are alternately stacked along a stacking directionperpendicular to the plane of the substrate, the manganese oxide thinfilm has two nonequivalent crystal axes along an in-plane direction ofthe plane of the substrate, and the manganese oxide thin film has, as abulk substance, a crystal lattice and a lattice constant defined by acubic root of a unit cell volume of the crystal lattice such that thelattice constant of the manganese oxide thin film is lower than thelattice constant of the crystal lattice of the substrate.
 10. An oxidelaminate, comprising: a substrate; a manganese oxide thin film formed ona plane of the substrate and having (a) a composition represented bycomposition formula RMnO₃, where R is at least one trivalent rare earthelement selected from lanthanoids, (b) an atomic layer containing anelement R and not containing Mn, and an atomic layer containing Mn andnot containing the element R which are alternately stacked along astacking direction that is perpendicular to the plane of the substrate,and (c) two nonequivalent crystal axes along an in-plane direction ofthe plane of the substrate; and a strongly-correlated oxide thin filmcontiguous to the manganese oxide thin film, wherein the oxide laminatehas a total thickness, t, the manganese oxide thin film has a thickness,tm, and the strongly-correlated oxide thin film has a thickness, t1,that satisfy a relation relative to a critical thickness, tc, underwhich the strongly-correlated oxide thin film has a metallic phase, asfollows:t=tm+t1>tcandt1<tc.
 11. An oxide laminate, comprising: a substrate; a manganese oxidethin film formed on a plane of the substrate and having (a) acomposition represented by composition formula RMnO₃, where R is atleast one trivalent rare earth element selected from lanthanoids, (b) anatomic layer containing an element R and not containing Mn, and anatomic layer containing Mn and not containing the element R which arealternately stacked along a stacking direction that is perpendicular tothe plane of the substrate, and (c) two nonequivalent crystal axes alongan in-plane direction of the plane of the substrate; a firststrongly-correlated oxide thin film contiguous to one surface of themanganese oxide thin film; and a second strongly-correlated oxide thinfilm that is contiguous to another surface of the manganese oxide thinfilm and that is comprised of the same material as that of the firststrongly-correlated oxide thin film, wherein the oxide laminate has atotal thickness, t, the manganese oxide thin film has a thickness, tm,and the first and second strongly-correlated oxide thin films haverespective thicknesses, t1 and t2, that satisfy a relation relative to acritical thickness, tc, under which the strongly-correlated oxide thinfilms have a metallic phase:t=tm+t1+t2>tcandmax(t1,t2)<tc, where max (t1, t2) is a function which returns a maximumvalue among variables.